Various processes for ceramic welding have been known since at least the 1930's using an exothermically reactive composition containing a major portion of relatively inert and noncombustible refractory material similar to or compatible with the brick lining of the oven or furnace to be repaired. The source of exothermic heat is provided by powdered metals, such as aluminum, magnesium and silicon. The combination of the powdered metal and refractory material as a dry powdery mix blown or "blasted" onto the hot oven walls with equipment which is similar to ordinary sand blasting equipment, generally requiring a high-pressure stream of oxygen to carry and blow the powdered material onto the place of damage where the oven is to be repaired. A long hollow wand is used by the operator to direct the material. The powdery material may ignite when it hits the hot surface of the oven or else, as recommended in earlier processes, supplementary flame ignition means are used.
The object of ceramic welding processes is to cause the reacted material to firmly adhere to the walls of the oven and provide a long lasting wall repair. The composition which fills the cracks or gaps in the damaged wall is altered by the heat of reaction to yield a repair material as physically and chemically similar to the original oven wall material as possible. The built up repair must be not only durable under normal furnace or oven operation, and must also have satisfactory refractory characteristics. The repair patch or renewed refractory lining should resemble the physical characteristics of fire brick normally used to line such ovens, i.e. so as to be somewhat porous rather than being so vitreous or glass-like as to break or crack under thermal stresses. At the same time, excessive porosity must be avoided to achieve a reasonably solid, brick-like consistency of the hardened or fused refractory material.